Propose minor icon improvements

Makes an attempt to improve the Yubiswitch icon by doing a minor redesign that includes retina friendly image sizes.

@pallotron, I'm not an expert on this by any means so feel free to take or leave this contribution. Here are a couple of notes about the change:

- I did the icon design in Sketch.app and included the .sketch file in case anyone wishes to make further modifications or improvements.
- I did have to update the `CFBundleIdentifier` to get notification center to pick up the new icon (sounds like a [known issue](http://stackoverflow.com/questions/11856766/osx-notification-center-icon)).
- It would be good to test on a non-retina mac.
- It looks like the [ShortcutRecorder submodule on master](https://github.com/Kentzo/ShortcutRecorder/tree/80ece17410b43e2ffe210779e1bab3b9cf3b56cd) is currently pointing to a sha that hasn't been pushed yet? Doesn't really affect anything in here, but just FYI.

## Disabled state

![off](https://cloud.githubusercontent.com/assets/136521/4695397/9d07810a-57b5-11e4-86b0-948da7996cb5.png)

## Enabled state
![on](https://cloud.githubusercontent.com/assets/136521/4695398/9fced186-57b5-11e4-900d-ad1b3d4b3a7c.png)